<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script type="text/javascript">
        var i;
        alert(i);//undefined
        i=12;
        //typeof()是Javascript语言提供的函数
        //它可以取变量的数据类型返回
        alert(typeof(i)); //number
        i="abc";
        alert(typeof(i));//String

        var a=12;
        var b="abc";
        alert(a*b);//NaN是非数字 非数值
    </script>
</head>
<body>

</body>
</html>